The positioning technology is one of important applications in next-generation communication systems. Although the positioning technology has already put to practical use at present, it is necessary to discuss a process for positioning a mobile terminal in a next-generation communication system which is capable of high-speed download communications.
As shown in FIG. 1, there is a process for positioning a mobile terminal using a radio base station apparatus based on a function for mobile radio terminal apparatus 3 to send back a signal that has been received from radio base station 4 through transmission path 100 (see, for example, JP-A No. 11-178038).
Positioning apparatus 6 is connected to radio base station 4 through communication control center 5. Positioning apparatus 6 measures a propagation delay time (phase) from a sent signal and a signal sent back from mobile radio terminal apparatus 3, and measures the distance between radio base station 4 and mobile radio terminal apparatus 3 based on the radio wave propagation speed.
Mobile radio terminal apparatus 3 comprises controller 31, transceiver 32, and speech unit 33 having sending-back means 34. Radio base station 4 comprises controller 41 and transceiver 42. Communication control center 5 comprises controller 51 and circuit interface unit 52. Positioning apparatus 6 comprises controller 61, phase measuring unit 62, and calculation processor 63.
However, the above mobile communication system with the positioning capability is problematic in that since no specific examples of the modulation process, etc. are not shown, there is not much information about how to operate the system in practice. Just like W-CDMA (Wideband-Code Division Multiple Access) used as the present third-generation mobile communication system, it is necessary for practical application to present a specific system for a certain scheme that has been established.
Furthermore, the above mobile communication system with the positioning capability is disadvantageous in that it tends to cause a propagation time error. Though there is no problem arising if transmission path 100 shown in FIG. 1 provides a clear obstacle-free environment, there are certain obstacles present in transmission path 100 in reality. Particularly in an urban area, shadowing occurs due to buildings or the like, and reflections from other directions reach radio base station 4 and cause multipath fading, making it difficult to position mobile radio terminal apparatus 3 along a straight distance.
To solve the above problems, there has been proposed, as a conventional mobile communication system with a positioning capability, a mobile terminal positioning system using an OFDM (Orthogonal Frequency Division Multiplexing) radio base station apparatus, as disclosed in Document 1 shown below.
Document 1: “The Institute of Electronics, Information and Communication Engineers, Technical Report RCS2001-32”, May 2001
A positioning process using a path search has been proposed as a process for reducing a propagation time error between a base station and a mobile terminal in a multipath environment, as disclosed in Document 2 shown below.
Document 2: “IEICE TRANSACTIONS ON COMMUNICATIONS”, Vol. E85-B, No. 10, October 2002
An article in Document 2 shows the system of a mobile radio terminal apparatus incorporating a path search for W-CMDA and OFDM. The system is configured as shown in FIG. 2.
FIG. 2 illustrates a system capable of measuring an arrival time difference between download positioning signals sent from different radio base stations (not shown). Mobile radio terminal apparatus 7 has receivers (#1, #2) 8, 9 which measure arrival times depending on download positioning signals from different radio base stations.
Receiver (#1) 8 has time signal replica generator 83 for generating a replica of a known pilot signal and correlators 82-1 through 82-N for calculating cross-correlated values of subcarriers. Delay profile generator 86 generates a delay profile based on the sum of output signals, which sum is calculated by adder 84. Delay profile generator 86 is set to a threshold value for a path search by a threshold value setting unit 85. First path detector 87 detects a first arrival path, and an arrival time calculator 88 measures an arrival time.
In FIG. 2, mobile radio terminal apparatus 7 is constructed of receivers (#1, #2) 8, 9 and arrival time difference calculator 10. Receiver (#1) 8 comprises S/P (Serial/Parallel) converter 81, correlators 82-1 through 82-N, time signal replica generator 83, adder 84, threshold setting unit 85, delay profile generator 86, first path detector 87, and arrival time calculator 88. Though not shown, receiver (#2) 9 is of the same construction as receiver (#1) 8.
Mobile radio terminal apparatus 7 calculates arrival times from respective two radio base stations, and produces output signals representative of the calculated arrival times from respective receivers (#1, #2) 8, 9. In mobile radio terminal apparatus 7, using the output signals from two receivers (#1, #2) 8, 9 which correspond to the respective radio base stations, arrival time difference calculator 10 can measure a relative arrival time difference between arrival times from the two radio base stations at mobile radio terminal apparatus 7. Details of a method of calculating the absolute position of mobile radio terminal apparatus 7 from the arrival time difference are revealed in Documents 1, 2, and will not be described below.
However, the above conventional mobile terminal positioning system as it is incorporated in the mobile radio terminal apparatus is problematic in that the mobile radio terminal apparatus tends to be complex and large in size. The tendency runs counter to efforts in recent years to reduce the size and cost of mobile radio terminal apparatus terminal.
Another problem of the conventional mobile terminal positioning system is that the accuracy of a path search is lowered depending on the propagation environment. If the SIR (Signal to Interference power Ratio) is low, then since the path is liable to be buried in noise, it is necessary to set a lower threshold value for path detection. Conversely, if the SIR is high, it is necessary to set a higher threshold value to prevent noise from being detected in error as a path.
More specifically, the conventional mobile terminal positioning system anticipates usage in an urban area or the like which is susceptible to shadowing, and is aimed at accurately measuring an arrival time of a signal between a radio base station apparatus and a mobile radio terminal apparatus in a multipath environment. The conventional mobile terminal positioning system employs a path search process for selecting a path that has arrived earliest as a first path, regarding the path as an arrival time, and calculating the distance between the radio base station apparatus and the mobile radio terminal apparatus.
For conducting a path search, it is necessary to determine a delay profile and set a threshold value for determining whether a path is found or not in order to detect a correlation peak. In the actual environment of a cellular phone radio network, however, received signals fluctuate and are attenuated by fading.
When the reception SIR is low, there is a tendency for a higher probability of burial of a path in noise. If the set threshold value is too high, then a path may not possibly be detected. Conversely, if the set threshold value is too low, then noise may easily be detected in error as a path. If the same threshold value is set as when the reception SIR is high, then it is difficult to detect a proper first path due to the propagation environment. In such a situation, the positioning accuracy is lowered because the arrival time of a signal between the radio base station apparatus and the mobile radio terminal apparatus is measured from the falsely detected first path.